Twisted nematic liquid crystal spatial light modulators have been widely used for such applications as optical information processing and real-time holography. It is important to experimentally determine the physical parameters of the modulators to control their modulation characteristics. The parameters are usually calculated from a set of measured intensity transmittances. We mathematically prove that the calculation always suffers from ambiguities, and there exist two distinct origins. Neglecting absolute phase is one source of ambiguity; the other relates to the quasisinusoidal dependence of the complex amplitude transmittance on the parameters. Jones matrix formalism is used to analyze the ambiguity, and interesting examples are presented. © 2002 Society of Photo-Optical Instrumentation Engineers.